Crosslinkers are multi-functional molecules capable of reacting with pendant functional groups on polymers. The use of crosslinkers enables one to increase the molecular weight of the resin or polymer and thus improve the properties of the resulting polymer or polymeric film. Most crosslinking reactions are initiated by heating a mixture of the polymer and the crosslinker either neat or in a solvent. Such systems are often referred to as "thermosetting" systems.
Crosslinkers are particularly useful in coating applications due to the fact that the crosslinker enables the use of relatively low molecular weight polymers and resins which are easily handled and applied in solvents. The formulation can subsequently be applied to the substrate and heated, or cured, to give the finished (thermoset) coating. This feature makes it possible to take advantage of the ease of handling and solubility characteristics of the lower molecular weight resins used in the formulation and subsequently develop the hardness, chemical and solvent resistance, as well as strength properties desired in the ultimate coating by the reaction of the crosslinker with the resin during the curing process.
Crosslinkers are becoming increasingly important due to the emphasis on more environmentally acceptable coatings. One major environmental concern in the coatings industry is the amount of organic solvent released during the curing process. This solvent level or Volatile Organic Content (VOC) is of concern due to the role of organic solvents in the development of photochemical smog. For these reasons various governments, including the U.S., are regulating the VOC levels of coating formulations. One way to reduce the amount of solvent necessary in a coating formulation is to reduce the molecular weight of the resin backbone used in the formulation. When this approach is used, however, crosslinking becomes even more critical to the development of the ultimate properties in the cured film.
Properties of Crosslinked Films and Coatings
A number of properties are desired in a coating in order to impart the desired protection of the object from corrosion and other environmental factors. Some of the protective characteristics that are ultimately desired include the resistance of the coating to various chemicals and solvents, the impact strength of the system, the hardness of the coating and the weatherability or resistance of the system to various factors related to environmental exposure.
I) Chemical and Solvent Resistance
In order for a coating to impart adequate protection to the object coated it must be resistant to various chemicals and solvents. If a coating is not resistant to solvents and chemicals the coating could be removed or the protective integrity compromised by exposure to commonly used materials such as industrial cleaners, gasoline, etc. A commonly used test to assay this property is the methyl ethyl ketone (MEK) rub resistance of the coating. The MEK rub resistance of a coating is often one of the most useful diagnostic tests for crosslinking in coatings. For most applications, a MEK rub resistance of greater than 175-200 is desired.
II) Impact Strength
In order for a coating to be resistant to collisions and other sudden impacts the material must have certain strength characteristics. If a coating does not possess enough strength, a physical impact with another object will lead to chipping and breaking of the coating which, in turn, compromises the protective integrity of the film. A commonly used test for the impact strength of a coating (ASTM D2794-84) is performed by dropping a weight from various heights on a coated panel and determining the foot-lbs. of force required to break the coating. Proper crosslinking can help develop the impact strength of a coating.
III) Hardness
In order for a coating to be resistant to scratching and other such abrasions the coating must possess a certain degree of hardness. This resistance to scratching is often determined by marring the coating with pencils of various hardness and noting which hardness of pencil actually scratches the coating.
Hardness and impact strength often work in opposite directions. This is due to the fact that impact strength reflects both the strength and the flexibility of the polymeric film, while hardness reflects primarily just the strength, or rigidity of the film. Thus, one often seeks a combination of hardness and flexibility by compensating one of the above characteristics for the other.
The compensation of these two factors is best understood by invoking the theory of crosslink density (see, for example, Vogel, H. A. and Bader, A. R., U.S. Pat. No. 2,730,517). If the coating formulation consists of a group of polyfunctional (n&gt;2) polymer molecules and crosslinker then the crosslinking process can be thought of as consisting of a series of steps. Initially, the crosslinking reaction consists of intermolecular reactions of various polymer chains. During this initial phase the chains are combining and thus building in molecular weight, but, the mobility of the polymer chains is not greatly restricted. This stage would be characterized by improvement in the chemical resistance, hardness and impact strength of the film. At some point, however, intermolecular reaction is essentially complete and intramolecular reaction becomes significant. At this point, the polymer becomes more rigid due to restriction of the polymer chain mobility by these intramolecular reactions and the resulting coating becomes more brittle. At this stage, hardness will improve but the impact strength will decrease due to the increased rigidity of the polymer network. The balance between flexibility and hardness can be controlled by the amount of crosslinker used, the average functionality of the polymer and crosslinker as well as the chemical structure of the polymer or crosslinker.
IV) Resistance to Atmospheric Exposure (Weathering)
Since many coated objects are exposed to severe weather conditions, the performance of the coating under various exposure conditions is very important. Factors which affect the weatherability of the coating include the composition of the polymer and the crosslinker, as well as the degree of crosslinking. A variety of exposure tests are available which enable one to determine the performance of the system to severe conditions.
Crosslinkers Currently Used in the Field
A large number of crosslinkers are used in various applications. A partial list of the more commonly types of compounds used as crosslinkers include:
Polyepoxides PA1 Polyisocyanates PA1 Amino resins (e.g. melamines) PA1 Polyunsaturated compounds PA1 (a) about 15 to 80 percent, based on the weight of the total composition of polyester or acrylic. PA1 (b) about 0 to 50 percent, based on the weight of the total composition of solvent. PA1 (c) about 10 to 40 percent, based on the weight of the crosslinker described above. PA1 RESIN A: This material was an acrylic resin prepared from 20 mol % hydroxyethyl methacrylate and 80 mol % methyl methacrylate and had a hydroxyl value of 106. The resin was used as a 60% solids solution in EEP. PA1 RESIN B: This material was a polyester prepared using a two-stage addition procedure from 2.47 moles neopentyl glycol, 0.78 moles trimethylolpropane, 1.73 moles 1,4-cyclohexanedicarboxylic acid, and 1.17 moles phthalic anhydride. The material had a Mw=12160, a Mn=4300, a hydroxyl value of 104, and an acid value of 9. This material was thinned with xylene and used as a 75% solids solution. PA1 RESIN C: Same as Resin B except a hydroxyl value of 103, an acid value of 4.5, a Mw=23082, and a Mn=2692. This material was thinned with xylene and used as a 75% solids solution. PA1 RESIN D: This material was a polyester prepared using a two-staged addition procedure from 2.30 moles neopentyl glycol, 0.86 moles trimethylolpropane, 1.44 moles isophthalic acid, and 1.44 moles adipic acid. The material had a Mn=2573, a hydroxyl value of 88, and an acid value of 3. This material was used as a 99.2% solids formulation.
These materials take advantage of the reaction of the aforementioned functional groups with various pendant groups on the polymeric backbone. These crosslinkers can be used in combination with other crosslinkers to impart a variety of desired characteristics to the coatings. The use and reactions of these crosslinkers have been reviewed elsewhere (see U.S. Pat. No. 2,730,517, incorporated herein by reference). All of these materials are structurally very different from the 2-acetyl glutarate esters of polyols disclosed in this invention. Bisacetoacetates of general formula 1 have been shown to act as a crosslinker and are the subject of U.S. Pat. No. 5,247,122. ##STR2##
It has also been shown that acetoacetylated polymers have undergone Michael reactions with acrylates and polyacrylates to form crosslinked coatings under various conditions (usually strongly basic). (See, for example, Clemens, R. J. and Rector, F. D.; Journal of Coatings Technology, Vol. 61, No 770, 83, (1989); Vogel, H. A. and Bader, A. R., U.S. Pat. No. 2,730,517; Bartman, B. and Swift, G., U.S. Pat. No. 4,408,018); Noomen, A., van Dongen, J. P. M., and Klinkenberg, H., European Patent Application 91200528.7; and U.S. Pat. No. 5,107,649.)
Finally, U.S. Pat. No. 4,795,787 describes the Michael addition products of monocarboxylic or dicarboxylic acid esters capable of undergoing Michael addition with compounds containing at least two double bonds and are taught to be useful as crosslinkers in coating systems utilizing amine-containing or hydroxyl-containing resins.